The present invention relates to an apparatus for optical storage and readout or retrieval of information on or from an optical recording medium such as an optical or magneto-optical disk.
In the art of data storage and readout, an optical recording and reproducing apparatus is known in which an appropriate area on an optical recording medium is irradiated by a laser beam produced by a laser source, and a light beam reflected by the irradiated area on the recording medium is detected by a suitable optical sensor. An optical data storage and readout apparatus of the type indicated above has a sophisticated optical head which is constructed to converge laser beams at a specified pont or spot on the optical recording medium, and to receive a reflected light from the irradiated spot on the medium, so that the reflected light may be detected by a light sensitive element. An example of an arrangement of the conventional optical head is illustrated in FIG. 35, wherein laser beams radiated by a laser source 200 are condensed into parallel rays by a collimator lens 202, and the parallel rays are transmitted to an objective lens 208, passing through a beam splitter 204 and a quarter-wave plate 206. The parallel rays incident upon the objective lens 208 are converged at a selected point on an optical recording medium 210. The light beam reflected by the recording medium 210 is passed through the objective lens 208 and the quarter-wave plate 206, and reflected by the beam splitter 204. The light beam reflected by the beam splitter 204 is transmitted through a convex lens 212 and a cylindrical lens 214, and finally received by an optical sensor 216. This optical sensor 216 comprises a plurality of light-sensitive elements, and is adapted to produce tracking and focusing signals, as well as data signals indicative of information recorded on the recording medium. More specifically described, the optical head incorporates a device for moving the objective lens 208 along its optical axis, based on the focusing signal, for adjusting the focal point of the laser beam emitted from the objective lens 208, so as to compensate for a variation in the surface position of the recording medium 210 in the direction parallel to the optical axis of the objective lens 208. Further, the optical head incorporates another device for moving the objective lens 208 along the surface of the recording medium 210, based on the tracking signal, for moving the focal point of the emitted laser beam along the medium surface, in order to aim the laser beam at a specified point within the width of the appropriate recording track on the medium, since the radial position of the tracks may be varied for some reason or other. Alternatively, the optical head may use a tracking mirror for adjustment of the emitted laser beam along the medium surface. In this case, the optical head requires a device for operating the tracking mirror.
In the known apparatus as indicated above, however, the time for an access to a selected recording track on the recording medium is not short enough for high-speed data storage and readout operations, because of the relatively large size and weight of the optical head which houses a large number of various optical components such as a laser source, collimator lens, roof mirror or beam splitter, quarter-wave plate, objective lens, convex lens, cylindrical lens and optical sensors, as previously indicated, and the optical head requires a massive rigid frame for supporting these optical components so as to maintain precise alignment relative to each other. Further, the optical head suffers a loss of light energy during propagation of the light through so many components. Moreover, the use of such a large number of optical components and the requirement of precise relative alignment of the components increase the cost of manufacture of the optical apparatus. The inconveniences addressed above are prominent, especially where the apparatus is adapted to aim laser beams at a plurality of spots on a recording medium.